Catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for polymerizing conjugated dienes. Advantageously, these catalyst systems are more active in aliphatic solvents, which are the solvents of choice for environmental reasons, than in aromatic solvents. They are highly stereospecific and can produce conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polydienes have a linear backbone structure and exhibit good green strength and excellent viscoelastic properties. The linear backbone structure is believed to provide better tensile properties, higher abrasion resistance, lower hysteresis loss, and outstanding fatigue resistance in rubber compounds. Therefore, these cis-1,4-polydienes are particularly suitable for use in tire components such as sidewall and tread.
Among various lanthanide-based catalyst systems for producing cis-1,4-polydienes, the most commercially useful includes a neodymium carboxylate as the lanthanide compound, either a trialkylaluminum or a dialkylaluminum hydride as the alkylating agent, and an alkylaluminum halide as the halogen source.
The use of dialkylaluminum hydrides as the alkylating agent is often preferred because less dialkylaluminum hydride—as compared to trialkylaluminum—is required to obtain a target molecular weight. This is primarily due to the fact that dialkylaluminum hydrides are better chain transfer agents than trialkylaluminums. Since the molecular weights of cis-1,4-polydienes produced by lanthanide catalysts generally increase with a decrease in the ratio of organoaluminum to lanthanide compound, less dialkylaluminum hydride can be used. Therefore, the use of dialkylaluminum hydrides offers the advantage of reducing the catalyst cost.
The use of dialkylaluminum hydrides, however, produces polymers that have a broad molecular weight distribution and contain a small fraction of ultrahigh molecular weight material. This unfortunately impacts the processing characteristics and viscoelastic properties of the polymers. Specifically, the broad molecular weight distribution generally results in higher hysteresis loss in rubber vulcanizates and therefore reduces fuel economy of tires. The ultrahigh molecular weight fraction causes high compound Mooney viscosity and high solution viscosity, both of which are undesirable. The high compound Mooney viscosity adversely affects the processability and scorch safety of rubber compounds, and the high solution viscosity is disadvantageous if the cis-1,4-polydiene is to be used in the production of high-impact polystyrene. Moreover, during the synthesis of cis-1,4-polydienes, the high solution viscosity causes difficulty in stirring and transferring the polymer cement and reduces the capacity for removing the heat of polymerization, which limits the polymer concentration that can be achieved in production.
Because the use of organoaluminum hydrides as the alkylating agent in lanthanide-based catalyst systems offers a significant cost advantage, there is a need to overcome the above-mentioned disadvantages associated with the use of organoaluminum hydrides.